Fix bitbucket login

WIP: The fix will be tested on staging today

What does this MR do?

It fixes the login using bitbucket

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The problem was cause by the update of omniauth-oauth2 which we updated in 10.1 release (https://gitlab.com/gitlab-org/gitlab-ce/commit/5d96b1489e57ee66e50b8ff5ea018264892c8185). As described in https://github.com/intridea/omniauth-oauth2/issues/81 in order to fix it callback_url has to be overrided.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39495 (closed)